Cyclo-oxygenases (COX) and their products are thought to be important in inflammation and nociception. COX isoforms are found in neurons thought to be nociceptive, and COX inhibitors alter neuronal processing of noxious inputs. We have used COX-2 knockout mice to study the role of this COX isoform in inflammation and nociceptive processing. Wild-type (n = 10) or COX-2 knockout (KO) mice (n = 8) were injected intradermally with 100 µl of Freund’s Complete Adjuvant (FCA, 2.5 µg µl-1 M. tub. in mineral oil) at two sites around the left tibiotarsal joint under brief inhalational anaesthesia (4 % halothane in O2 ). Mice were monitored for 20 days for paw swelling, mechanical allodynia using von Frey filaments, and thermal hyperalgesia. Animals were humanely killed on day 20 by cervical dislocation. All procedures were in accordance with UK legislation. Paw circumferences were compared using ANOVA. Changes in nociceptive behaviours were compared with baseline using one sample t tests. All data shown are means ± S.E.M.
All animals showed a rapid ipsilateral paw swelling apparent within 24 h that was maintained throughout the 20 days of the study (WT: 138 % control, KO: 124 % control at 24 h; P < 0.01). The contralateral hindpaws showed no swelling at any point in the study. There was no significant effect of genotype on ipsilateral or contralateral joint swelling.
COX-2 KO mice had significantly lower mechanical withdrawal thresholds compared with WT mice before FCA injection (WT: 1.6 ± 0.04 g; KO: 1.1 ± 0.04 g, P < 0.05). Unilateral inflammation resulted in a small but significant reduction in von Frey thresholds in WT mice at days 1, and 8 to 17 post-injection. Von Frey thresholds in KO mice were unaltered by inflammation at any point.
COX-2 KO mice had significantly longer withdrawal times to noxious thermal stimulation compared with WT mice before FCA injection (WT: 8 ± 0.2 s; KO: 11 ± 0.2 s, P < 0.01). Neither WT nor KO mice developed more pronounced thermal hyperalgesia in either the inflamed or the contralateral limb at any point after FCA injection.
Knockout of COX-2 in mice rendered the animals more sensitive to mechanical stimuli and less sensitive to thermal stimuli. In inflammation KO mice do not show altered mechanical nociceptive thresholds, unlike WT animals. These data suggest that COX-2 is involved in normal somatosensory processing of mechanical and thermal stimuli and that COX-2 may be involved in mechanical nociceptive processing in FCA-induced inflammation. In addition, inflammation and swelling were unaffected in COX-2 KO mice, suggesting that swelling seen in this model is not COX-2 dependent.
This work was funded by the Arthritis Research Campaign, UK.